A patient bed required in magnetic resonance imaging equipment is in a per se known manner used not only to support a patient in a position required by imaging but also as a platform for the accessories needed during an imaging process. The most important of such accessories or facilities is an rf-coil, required for receiving an NMR (nuclear magnetic resonance) signal originating from a patient. Sometimes the same coil is also used for transmitting an excitation pulse required for producing a signal. This is often effected by means of a separate transmission coil which can be placed either inside the magnet or on the patient's bed. Other accessories include e.g. various cushions, supports and attachment belts for a patient's comfort and a proper imaging position. In addition, a patient bed must be capable of performing a variety of positionings: positioning of a patient and a coil on the bed, positioning of a patient relative to the coil, and positioning of a patient as well as a coil relative to the magnet.
In order to optimize a signal-to-noise ratio, the rf-coils required for reception must be designed so as to surround an object to be imaged with as high a degree of filling as possible. Thus, various anatomical targets, such as the head, the neck, the knee etc., must be provided with their own separate coils which are placed on a patient bed as close to the target or object as possible or around the object.
The coils required in transmission are generally bulky in size relative to the object in order to achieve a sufficient transmission field homogeneity. Often it is sufficient to have a single common transmission coil, which is usually placed inside the magnet and which is fixed in position and detached from the patient bed. It is also possible to employ separate transmission coils, and in some cases, the transmission and reception coils can be combined to provide a single coil assembly. In these cases, the coil is positioned into the center or bore of the magnet by utilizing the bed or its platform.
A patient is positioned on the bed most often in a lying position although e.g. a wrist can be imaged by having a patient in a sitting position at the inlet opening of a magnet on a lowered bed while holding his or her hand inside the magnet. When in a lying position, a patient is usually lying on his or her back. When imaging the head and the neck, a patient is generally pushed into the magnet head first and, when imaging the lower limbs, generally feet first. Otherwise, the orientation can be freely selected, although a patient is generally pushed into the magnet head first. Positioning of a patient on the bed can be facilitated by lowering the bed for a patient to settle thereon.
The reception coil is set in position either before or after a patient is transferred onto the bed. It is necessary to preset such coils which are difficult or impossible to position afterwards relative to a patient. Such coils include e.g. a spine coil positioned underneath a patient or a neck coil placed under the neck. A drawback in these coils is that, if a coil is to be replaced, the patient must move out of the way for a replacement. In addition, the coil may get in the way during the transfer of a patient. Such coils are mounted on the bed either at a single spot or at several alternative, generally permanent locations.
It is possible to position afterwards the coils to be mounted on top of a patient or to be threaded into position or the coils to be drawn around a patient. Such coils include e.g. a heart coil to be placed on top of the thorax, a shoulder coil to be threaded into the axilla, and a head coil to be drawn around the head.
Thus, some coils can be designed in a manner that a segment of the coil is pre-mounted on the bed as a patient moves onto the bed and on top of said segment of the coil. The other coil segment is after that placed in position. This type of coil can be e.g. an openable head-, body- or knee coil. A drawback in such coils is that there is usually required an electric contact between the different segments and this can be electrically delicate or mechanically complicated to carry out. The openability may also limit the geometry of a coil. In addition, the coil segment pre-mounted on the bed may also get in the way during the move-over of a patient.
For the imaging process a patient must be positioned relative to the coil in a manner that a spot to be imaged will be located within the field of imaging as well as possible. The center of a field of imaging is generally the center of symmetry of a coil and it can be indicated by means of auxiliary markings. In order to achieve a proper positioning, the relative location of a patient and a coil must necessarily be changed, an the same applies if the location of a field of imaging should be changed during an imaging session. With fixedly positioned coils this may be inconvenient, particularly if a patient is difficult to move over. Especially spine and body imaging processes can be problematic. In spine imaging, despite a fixedly positioned casing, the imaging coil itself can be moved within the casing relative to a patient. However, this can lead to inconvenient mechanical solutions and and result in an impractically large-size coil box or casing if the mobility is to be extended over the entire spinal area.
The positioning is generally carried out with the help of tracer 8 of (FIG. 1), which indicated e.g. the location of the center of a coil. If the location of a coil on the bed is arbitrary, the coil must be provided with some kind of marking to indicate the location of the center of said coil. The marking may sometimes be hidden underneath a patient. The positioning tracers are generally located at the mouth of a magnet, which in view of a practitioner can be a cramped spot for inspecting the positionings.
As for the transmission coils, a fixed transmission coil fitted inside a magnet is the handiest alternative in view of handling a patient and the coil. If, for example due to a higher transmission field amplitude, it should be desirable to employ a smaller transmission coil, the electrical elimination of a fixed transmission coil may cause trouble. In this case, a removable transmission coil is a preferred solution. In this case, for example, a transmission coil can be inserted inside a magnet the back way, the patient's bed extending through the coil. Hence, the bed can be telescopic which sets special requirements in terms of rigidity. The bed and its platform can also be supported at its rear end, whereby a transmission coil runs within a recess underneath the bed and cleaning of such recess may prove to be a problem.
When a patient has been positioned relative to a coil, the coil and the patient must still be brought in the middle of a magnet in a manner the imaging center of a coil will be located in the center 9 (FIG. 1) of a magnet at least in longitudinal direction. If the location of a coil on the bed varies, it is necessary to employ some type of scale of length and its numerical system for bringing a coil in the middle of a magnet. An auxiliary marking on the coil and positioning tracers are generally used as a means for informing the system of the location of a coil on the bed. The numerical system of the scale is also utilized for stopping a bed relative to a magnet in a manner that the coil will be positioned in the middle of the magnet. If the magnet has a sufficiently open structure, a coil can be located right in the middle of the magnet with the positioning of a patient relative to the coil being also effected there. If the magnet is provided with a sufficiently large bore, the bed can also be moved laterally in addition to longitudinal direction.
If the location of a coil on the bed is arbitrary, the coil cables will generally be long since they must be dimensioned according to the furthest-away location whenever the bed has been pulled out of a magnet. During the movement of a bed the long cables may readily get stuck e.g. between magnet and bed unless this has been prevented by means of some kind of additional structure or unless the equipment operator guides the cables.
Generally, a single imaging apparatus is operated by employing a plurality of coil positioning techniques. This may complicate the mechanical structure of the apparatus and restrict the introduction of new types of coils. In addition, the practitioners or nurses operating the apparatus must be familiar with several different operating and positioning techniques, which is a possible cause of malfunctions especially in emergencies. A complicated structure may also result in poorer maintenance of the apparatus.